Charge imbalance in superconductors in the low-temperature limit

TL;DR

This study investigates charge imbalance phenomena in mesoscopic superconductor-normal metal structures at very low temperatures, revealing details about relaxation lengths, scattering mechanisms, and magnetic field effects relevant for non-local quantum effects.

Contribution

It provides detailed measurements of charge imbalance and relaxation mechanisms in low-temperature superconductor-normal metal structures, highlighting the effects of magnetic fields and bias voltage.

Findings

Charge-imbalance relaxation length lambda_Q* derived from conductance data.

Transition from elastic to inelastic relaxation with increasing bias.

Strong suppression of charge imbalance by magnetic field, minimal temperature dependence below 0.5 K.

Abstract

We explore charge imbalance in mesoscopic normal-metal/superconductor multiterminal structures at very low temperatures. The investigated samples, fabricated by e-beam lithography and shadow evaporation, consist of a superconducting aluminum bar with several copper wires forming tunnel contacts at different distances from each other. We have measured in detail the local and non-local conductance of these structures as a function of the applied bias voltage V, the applied magnetic field B, the temperature T and the contact distance d. From these data the charge-imbalance relaxation length lambda_Q* is derived. The bias-resolved measurements show a transition from dominant elastic scattering close to the energy gap to an inelastic two-stage relaxation at higher bias. We observe a strong suppression of charge imbalance with magnetic field, which can be directly linked to the pair-breaking parameter. In contrast, practically no temperature dependence of the charge-imbalance signal was observed below 0.5 K. These results are relevant for the investigation of other non-local effects such as crossed Andreev reflexion and spin diffusion.